For optical fibers, the refractive index profile is generally qualified in relation to the graphical representation of the function that associates the refractive index with the fiber radius. Conventionally, the distance r to the center of the fiber is shown along the abscissa, and the difference between the refractive index and the refractive index of the external cladding of the optical fiber along the ordinate axis. The refractive index profile, therefore, is referred to as “step,” “trapezoidal,” or “triangular” profile for graphs having the respective shapes of a step, trapezoid, or triangle. These curves are generally representative of the theoretical or set profile of the fiber. The manufacturing constraints of the fiber, however, may lead to a slightly different profile.
An optical fiber conventionally consists of (i) an optical core whose function is to transmit and possibly to amplify an optical signal, (ii) an optical cladding whose function is to confine the optical signal within the core, and (iii) an external cladding having a substantially constant refractive index ng. For this purpose, the refractive indices of the core nc and of the external cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber decomposes into a fundamental mode guided in the core and into secondary modes, called cladding modes, that are guided over a certain distance in the core-cladding assembly.
In the new, high bit-rate, wavelength-multiplexed transmission networks it is advantageous to manage chromatic dispersion, particularly for rates of 10 Gbits/s or higher. The objective, for all multiplex wavelength values, is to achieve a cumulated chromatic dispersion that is substantially zero on the link in order to limit pulse broadening. “Cumulated chromatic dispersion” is the name given to the integral of chromatic dispersion over the fiber length. When the chromatic dispersion is constant, the cumulated chromatic dispersion is equal to the product of chromatic dispersion and the length of the optical fiber. A cumulated value of a few dozen picoseconds per nanometer (ps/nm) for dispersion is generally acceptable. Near the wavelengths used in the system, it is also advantageous to avoid zero values of local chromatic dispersion for which non-linear effects are stronger. Finally, it is also advantageous to limit the cumulated chromatic dispersion slope over the multiplex range so as to avoid or limit distortions between the multiplex channels. This slope is conventionally the derivative of chromatic dispersion with respect to wavelength.
As line fibers for land transmission systems, single-mode fibers (SMF) or Non-Zero Dispersion Shifted Fibers (NZDSF) are conventionally used. NZDSF+ are dispersion shifted fibers having a non-zero, positive chromatic dispersion for the wavelengths at which they are used, typically around 1550 nm. For these wavelengths, these fibers have low chromatic dispersion, typically lower than 10 picoseconds per nanometer-kilometer (ps/(nm·km)) at 1550 nm, and a chromatic dispersion slope typically lower than 0.1 ps/(nm2·km).
To compensate chromatic dispersion and the chromatic dispersion slope in SMF or NZDSF+ used as line fibers, short lengths of Dispersion Compensating Fiber can be used (DCF); the fiber then has a negative chromatic dispersion and a negative chromatic dispersion slope. For the choice of DCF fiber, it is generally sought that the ratio of the chromatic dispersion over the chromatic dispersion slope of the chromatic dispersion compensating fiber is substantially equal to that of the line fiber. This ratio is designated by the abbreviation DOS for Dispersion Over Slope ratio.
U.S. Pat. No. 5,568,583 or U.S. Pat. No. 5,361,319 describes DCF fibers for compensating the chromatic dispersion of SMF fibers, and EP-A-1,067,412 describes a DCF for compensating the chromatic dispersion of NZDSF+s. These known DCFs, at a wavelength of 1550 nm, exhibit a negative chromatic dispersion and a negative chromatic dispersion slope.
Optical systems that are wavelength multiplexed, called Wavelength Division Multiplexing (WDM), generally consist of a concatenation of line fiber sections—SMF, NZDSF+ or others—with dispersion compensating modules inserted between the line fiber sections and comprising spooled DCF sections. The manner in which the dispersion compensating modules are distributed along the transmission line is called dispersion management; the objective of this management is to limit both non-linear effects and cumulated end-of-line dispersion. It is always sought, at the end of the line, to achieve a low cumulated chromatic dispersion and a zero cumulated chromatic dispersion slope.
In this context, the “transmission line section” refers to a part of an optical transmission system linking a transmitting element to a receiving element, these elements possibly being located at the end of the line or in nodes of the optical system. A line section, therefore, includes one or more concatenated line fiber sections and one or more sections of dispersion compensating fiber distributed between the sections of the line fiber. The line fiber sections usually generate a positive chromatic dispersion with a positive chromatic dispersion slope, whereas the sections of dispersion compensating fiber generate a negative chromatic dispersion with a negative chromatic dispersion slope. In the event of overcompensation, the line section will therefore exhibit a negative cumulated chromatic dispersion with a negative cumulated chromatic dispersion slope that has to be compensated for in order to arrive at zero dispersion at node entry or at the end of the line.
It is sometimes advantageous to insert an over-compensation along the transmission line, for example, to limit non-linear effects in the line fiber. It has also been found that overcompensation of the chromatic dispersion reduces the error rate at the receivers. For example, the article “Investigation of Advanced Dispersion Management Techniques for Ultra-Long Haul Transmissions” by J. C. Antona, M. Lefrancois, S. Bigo, and G. Le Meur, presented in September 2005 to the ECOC'05 Conference (European Conference for Optical Communications) indicates that overcompensation during transmission, illustrated in the article by a residual dispersion per subdivision or per negative line fiber section, makes it possible to improve the performance of WDM systems at 10 Gb/s. At the end of the line and/or at each node of the transmission system, however, the cumulated chromatic dispersion must be restored to zero or slightly positive. Yet, if the optical signal has been overcompensated, at the end of the line, the chromatic dispersion and the chromatic dispersion slope will be negative; it is then necessary, in order to offset this overcompensation, to use a fiber piece having a positive chromatic dispersion and a positive chromatic dispersion slope. For this purpose, sections of standard SMF (SSMF) or of Pure Silica Core Fibers (PSCF) are often used.
The major drawback with the use of a SSMF section to offset overcompensation is that the SSMF induces high losses with respect to the quantity of dispersion produced. This characteristic is generally determined by what is called the “Figure of Merit” (FOM). Figure of Merit is defined as the ratio of the chromatic dispersion D, in absolute value, to the attenuation of the signal in dB/km. For a SSMF, the FOM value is in the order of 85 ps/nm/dB. The PSCFs induce fewer optical losses and have a FOM value in the order of 125 ps/nm/dB, but they are costly.
In addition, as undersea fiber for intercontinental optical links, negative non-zero dispersion-shifted fibers are used, also called NZDSF−. Fibers qualified as NZDSF− are dispersion shifted fibers having a non-zero, negative chromatic dispersion for the wavelengths at which they are used, typically around 1550 nm. At these wavelengths, these fibers exhibit a low chromatic dispersion, typically lower than −2 ps/(nm·km) at 1550 nm, and a chromatic dispersion slope that is typically lower than 0.1 ps/(nm2·km).
To compensate the chromatic dispersion and the chromatic dispersion slope in NZDSF−s used as undersea lines, Positive Dispersion Compensating Fibers (P-DCF) must be used. Up until now, in marketed and installed undersea transmission systems, portions of SSMFs have been used to compensate the negative dispersion of NZDSF−s, whether in-line (cabled P-DCF), at the transmitter, or at the receiver (module P-DCF). However, as indicated above, the SSMFs have a FOM value that is too low for module use. PSCFs may also be used, but they are costly.
U.S. Pat. No. 6,337,942 proposes a positive chromatic dispersion compensating fiber to compensate an NZDSF. This optical fiber has a structure with a depressed cladding adjacent the central core and an external optical cladding. The central core can be doped with germanium or pure silica. The fiber disclosed in U.S. Pat. No. 6,337,942 exhibits a strong positive chromatic dispersion, of between 18 ps/(nm·km) and 21 ps/(nm·km) for transmission losses in the order of 0.2 dB/km, which leads to a FOM of 105 ps/nm/dB or lower. The presence of a depressed cladding adjacent the central core, with a strong refractive index difference, makes it possible to increase the chromatic dispersion while limiting the increase in the cut-off wavelength, but it also has the effect of increasing the transmission losses. To limit these losses, U.S. Pat. No. 6,337,942 also proposes increasing the diameter of the depressed cladding, which is between 36 microns and 46 microns, for a central core whose diameter is between 9 microns and 10 microns. A depressed cladding doped with fluorine, however, is costly to manufacture.
U.S. Pat. No. 6,665,482 describes a transmission fiber having a pedestal structure with a central core, a first positive inner cladding, and an external optical cladding. This fiber has an effective surface area of more than 90 μm2 at a wavelength of 1550 nm, making it possible to reduce non-linear effects and, hence, to increase the operating margins of wavelength multiplexed high bit-rate transmission networks. The increase in the fiber's effective surface area also leads to an increase in the positive chromatic dispersion compared with a SSMF, but it is intentionally limited to 20 ps/(nm·km) to avoid network penalties due to high cumulated dispersion values (before compensation). The transmission losses in the order of 0.2 dB/km lead to a FOM value of 100 ps/nm/dB or lower.
EP 1,255,138 describes a positive dispersion optical fiber having a large effective area, the fiber having a doped core region, a first annular region, a second depressed annular region, and a cladding region. The second depressed annular region has a normalized index difference between −0.08 and −0.20 Δ%, and an absolute index difference between −1.16 and −1.9×10−3 when calculated with respect to the index of silica.
U.S. Pat. No. 6,685,190 describes a fiber having an effective surface area of more than 110 μm2 at a wavelength of 1550 nm, enabling a reduction in non-linear effects and hence an increase in the operating margins of wavelength multiplexed high bit-rate transmission networks. The increase in the effective surface area of the fiber also leads to an increase in the positive chromatic dispersion, with a dispersion of between 18 ps/(nm·km) and 23 ps/(nm·km) for transmission losses in the order of 0.17 dB/km which leads to a FOM value of between 105 ps/nm/dB and 135 ps/nm/dB.
Despite higher positive chromatic dispersion values and improved FOM values compared with SSMFs, the fibers described in U.S. Pat. No. 6,658,190 are not well adapted for compensating the negative chromatic dispersion of an undersea NZDSF, or for offsetting overcompensation in a node of a land communication system. Indeed, an increase in the surface area leads to an increase in bending and microbending losses. Yet a DCF is intended to be rolled up in a housing of an optical dispersion compensating module in which fiber portions are superimposed; the fiber must therefore have limited bending and microbending losses for standard coatings and diameters (namely a naked fiber diameter of around 125 microns, a fiber diameter with first coating of around 200 microns, and a fiber diameter with second coating of around 250 microns).
None of the profiles of the prior art fibers cited and analyzed above makes it possible to obtain an optimal compromise between an increased FOM and acceptable characteristics in terms of bending and microbending losses for a positive chromatic dispersion compensating fiber (P-DCF).